The present invention relates generally to vehicle navigation systems and more particularly to a system and method for calibrating accelerometers in the vehicle navigation system.
Some known vehicle navigation systems include inertial sensors or accelerometers, possibly in combination with other sensors, to provide dead-reckoning. One known navigation system includes an accelerometer suite comprising three orthogonally mounted accelerometers. The accelerometer provides acceleration signals in three mutually orthogonal axes. A first accelerometer is aligned vertically, i.e., with gravity. A second accelerometer is aligned with the longitudinal axis of the vehicle to provide a signal indicating forward and rearward acceleration, from which speed can be determined. A third accelerometer, mounted orthogonally to the other two, provides lateral acceleration information, which is used to indicate change in heading.
In the known navigation system, the accelerometers must be precisely aligned with the associated axes of the vehicle in order to provide accurate information. The accelerometer is mounted in a known orientation in the vehicle. Hardware, such as adjustment screws, is provided to fine tune the orientation of the three accelerometers to align the three accelerometers with the corresponding axes of the vehicle. Even after time-consuming adjustment, the accelerometers may not be properly aligned with the axes of the vehicle. The alignment may also change over time.
The present invention permits a multi-axis accelerometer suite to be installed in the vehicle in any unknown orientation. The navigation system learns the orientation of the accelerometer suite and propagates the position of the vehicle based upon the acceleration signals from the accelerometer after learning its orientation. The navigation system determines the orientation of the accelerometer relative to the vehicle and generates a transformation matrix. The transformation matrix transforms signals from the accelerometer suite reference frame to the vehicle reference frame. The transformed acceleration signals are then used to provide dead reckoning. More particularly, a forward acceleration signal is used to determine the speed of the vehicle, while a lateral acceleration signal indicates change in the current heading of the vehicle.
Generally, the accelerometer is installed into the vehicle in any orientation. Preferably, the vehicle is then run through a series of simple, pre-determined actions which permit the navigation system to quickly determine the orientation of the accelerometer relative to the vehicle. First, when the navigation system determines that the vehicle is stationary, such as by monitoring change in acceleration signals, the navigation system determines the orientation of the accelerometer relative to gravity. Since the only acceleration acting upon the accelerometer when the vehicle is still is gravity (at 1 g), the navigation system can quickly determine the orientation of the accelerometer relative to gravity.
Next, the vehicle undergoes a series of moderate forward accelerations, which indicate the orientation of the accelerometers relative to the longitudinal axis of the vehicle. Since acceleration due to gravity has already been determined and since there can be no lateral acceleration without forward velocity, any acceleration in the forward direction of the vehicle before significant velocity is achieved is almost completely in the longitudinal axis of the vehicle. In this manner, the orientation of the accelerometer relative to the longitudinal axis of the vehicle is determined.
The lateral axis of the vehicle is then taken to be the cross product of the gravity and longitudinal axes of the vehicle. DC biases of the accelerometer, in each axis, are also determined. The orientation of the accelerometer is repeatedly checked, and updated, if necessary.